Titanium Part 6 doc

140 3 Technological Aspects Table 3.5. Metallographic etchants for titanium and titanium alloys Application Method Etchant Composition Kroll’s Etch Swab until surface becomes less reflective 95 ml H 2 O, 3 ml HNO 3 , 2 ml HF or 95 ml H 2 O, 4 ml HNO 3 , 1 ml HF Oxalic Acid Stain Etch Immerse and remove after polished surface appears “cloudy” Equal parts of aqueous 10% Oxalic acid and 1% aqueous HF solutions a b Fig. 3.80. Presence of microtexture in Ti-6Al-4V, LM: (a) Bright field (b) Polarized light Fig. 3.81. Nomarski interference image of a deformed Ti-Mo alloy showing the presence of twins, LM 50µm 30µm 30µm 3.9 Characterization Methods 141 3.9.2 Electron Microscopy Examination of titanium or its alloys using electron optic devices such as the transmission electron microscope (TEM) and the scanning electron microscope (SEM) provides complementary information to that obtained from other charac- terization methods such as light microscopy and X-ray diffraction. This section will outline the use of the TEM and the SEM in the characterization of titanium and its alloys including a brief summary of the relevant experimental techniques. 3.9.2.1 Transmission Electron Microscopy The ability to examine titanium and titanium alloys by thin foil transmission elec- tron microscopy lagged behind similar capability for other metals such as Al and Cu alloys. This was largely because of the difficulty encountered in preparing high quality thin foils of titanium base materials. This was in part related to the reactiv- ity of titanium including its propensity to absorb hydrogen during electrolytic polishing and chemical polishing. There are some early examples in the literature of hydrogen-related artifacts. Blackburn [2.26] was among the first investigators to demonstrate the routine preparation of artifact-free titanium thin foils by elec- trolytic polishing. He used the window method described elsewhere [3.39] along with the methanol, butanol and perchloric acid electrolyte and polishing condi- tions described earlier in Table 3.4. This polishing method is capable of preparing thin foils from a wide range of titanium alloy types and compositions and, if the polishing is done cold, minimizes the amount of hydrogen that is absorbed by the specimen. Today, jet polishing of 3 mm diameter disks using semi-automatic polishers explicitly designed to make TEM specimens is the preferred method of thin foil preparation. Jet polishing is fast, gives more reproducible results and lessens the risk of preparation-induced deformation in the thin sections being examined. The early commercial jet polishers were made by Fischione in USA, but Struers and other metallography equipment makers now sell jet polishers. The biggest drawback of electrolytic polishing is the local rate of removal which is strongly influenced by the local electrochemical dissolution potential. In two phase alloys that have individual constituents of significantly different chemi- cal composition, highly preferential attack of one constituent can occur. This can result in very nonuniform thinning or, in extreme cases, in only one constituent in the thin section. Thin foil preparation by ion milling eliminates this selective at- tack. Ion milling also has the advantage of not introducing hydrogen into the specimen, and may actually cause the hydrogen content to be reduced. This is because the ion mill exposes a pristine titanium surface in a vacuum under condi- tions of modest local ion beam heating. This method takes several hours minimum to produce a thin specimen, but it still has become popular because it is used for specialized situations, two of which will be described later (interface phase and spontaneous relaxation). Ion milling uses a beam of Ar ions accelerated by a high voltage (usually 10-20 kV) to sputter materials from the surfaces of a 3 mm di- ameter TEM disc. In materials with atomic numbers equal to or greater than tita- nium, there is minimal damage introduced into the specimen by the ion beam 142 3 Technological Aspects bombardment. Careful examination of titanium base specimens permits detection of some mottling due to the ions, but this does not interfere with forming good images and appears not to adversely affect the ability to examine the microstruc- ture of the specimen. The principle of thin foil preparation is the same no matter what material re- moval method is used. The material is gradually removed until the specimen is perforated. Along the edge of the perforation is a wedge of material that is thin enough to be electron transparent. The volume of material that is actually exam- ined in a TEM specimen is extremely small, but the high magnifications used for the examination sometimes cause the investigator to overlook this point. The use- fulness of TEM for characterizing uniformly distributed microstructural features is unsurpassed. For nonuniform features such as grain boundaries in large grain size materials, multiple foils typically must be prepared before the feature of interest is captured in the thin region. This is time consuming and can be very tedious. Higher accelerating voltage microscopes extend the amount of electron transpar- ent material in a foil, but this still is of relatively limited help if the features of interest truly are inhomogeneously distributed. Now a new device is available that allows thinning of TEM specimens at prede- termined sites. This device is a focused ion beam device (known as a FIB). The FIB was originally developed for electronic materials studies where the defect density is very low (< 10 4 /cm 3 ). It is now clear that the FIB is a useful tool for metallurgical studies also. The FIB transforms the TEM from a powerful charac- terization tool for general microstructural features to a problem solving tool as well. This is because this device permits areas of particular interest to be selec- tively examined because the thin section can be deliberately located at any desired position within the material being examined. The use of FIB devices in metallurgy studies is just beginning, but this instrument promises to revolutionize microstruc- tural studies in real structural materials of practical interest. This is especially true for materials containing inhomogeneously distributed microstructural features that may affect the performance of the material. During the early TEM studies of titanium and titanium alloys, several thin foil artifacts were common, but these were not always recognized as such. In addition to the gross effects of hydrogen on obscuring the microstructure mentioned earlier, there is a second effect worth mentioning. This is the formation of an interface phase in α+β alloys and the complete distortion of α precipitates in β alloys. These effects have been described in several publications [3.40, 3.41]. The inter- face phase is illustrated in Fig. 3.82. It now is clear that this phase is a thin foil artifact. This clarification was demonstrated by jet polishing and ion thinning specimens from the same material sample. The jet polished specimens contained the interface phase whereas the ion thinned specimens did not. This result is con- vincing because it shows that the interface phase is a hydrogen-related artifact. In reality, once this is understood, the interface phase does not cause any particular confusion in the basic interpretation of α+β titanium alloys microstructures. How- ever the interface phase does obscure the structure of the alpha/beta interface itself as can be seen in an ion-milled specimen, Fig. 3.83. In those cases where the inter- face is the object of study, ion milling is essential. If the electrolyte temperature is 3.9 Characterization Methods 143 allowed to get above about –20°C, there is the risk of significant hydrogen pickup during polishing. This can lead to sufficiently high bulk concentrations that hy- dride (TiH 2 ) precipitation can occur. These hydrides can form as plates, giving rise to displacement fringe contrast that can be mistaken for stacking faults. An exam- ple of this fringe contrast due to hydrides is shown in Fig. 3.84. Heating speci- mens containing such features to a few hundred degree Celsius in the microscope causes these hydrogen related features to disappear. Hydrogen related effects are the only reasonable explanation for these changes that occur over such a small temperature range. Polishing under conditions where the electrolyte is too warm also can result in surface hydrides that are small and coherent and which cause strain contrast. An example is shown in Fig. 3.85. These surface hydrides also disappear during heating, which is consistent with their identification as hydrogen related features. Fig. 3.82. Interface phase in an electropolished Ti-6Al-4V specimen, TEM Fig. 3.83. Ion milled specimen showing absence of interface phase, TEM (courtesy D. Banerjee, DMRL) 144 3 Technological Aspects Fig. 3.84. Displacement fringe contrast due to the presence of thin hydrides in Ti-5Al-2.5Sn, TEM Fig. 3.85. Strain contrast around electrolytic polishing induced surface hydrides along thin edge of the specimen, TEM Another characteristic of TEM examination of titanium alloys is the occurrence of “spontaneous relaxation” of metastable phases when the bulk constraint is re- moved during thinning. There are two examples worth noting. First is the trans- formation of orthorhombic martensite to a face-centered cubic structure in thin foils [3.42]. The second is the spontaneous shearing of metastable beta phase in the thin regions of the foil, leaving “martensite-like” features in the image that can be mistakenly thought to be part of the real microstructure [3.43], as shown in Fig. 3.86. Ion milled specimens containing orthorhombic martensite or metastable β phase, do not exhibit these two thin foil artifacts [3.44]. This article suggests that the “spontaneous relaxation” occurs by movement or creation of interfaces in the thin specimens and that a small amount of damage from the ion milling pre- vents this from happening. This is another advantage of ion milling. 3.9 Characterization Methods 145 Fig. 3.86. Example of thin foil artifact: “Martensite-like” features in metastable β phase, Ti- 6246, TEM 3.9.2.2 Scanning Electron Microscopy The intrinsically fine microstructural scale of titanium alloys requires examination at relatively high magnifications. This, coupled with some of the difficulties in mechanical polishing of titanium alloys, presents a challenge to prepare specimens with flatness of field suitable for obtaining high quality light micrographs at high magnifications. Consequently, the use of the SEM for metallographic examination at higher magnifications has become common. The high depth of field and good resolution of the SEM make examination of electrolytically polished samples easy and the results are very good. An example of a SEM micrograph is shown in Fig. 3.87 which gives a level of detail that would be very difficult to reveal by LM. In addition to electrolytic polishing the sample, etching with the more con- centrated (95-3-2) Kroll’s etch (see Table 3.5) provides good differentiation be- tween constituents. The excellent depth of field of the SEM also has allowed the development and use of another interesting technique for characterizing the relation between frac- ture topography and microstructure. In this technique, a portion of the fracture surface is preserved by masking using standard electroplating masking lacquer. The adjacent fracture surface is then electrolytically polished until it is smooth. The specimen is then etched using again the 95-3-2 version of Kroll’s etch. The lacquer is then removed with solvent and the fracture surface and the adjacent polished surface are cleaned and dried. When the sample is placed in the SEM, the intersection of the microstructure and the preserved fracture surface can be viewed directly. Both the microstructure and the fracture surface are simultaneously in focus in the SEM image because of the depth of field of the SEM. This technique permits the effects of microstructure on fracture topography to be directly ob- served. Two examples of the application of this technique are shown in Figs. 3.88 and 3.89. This technique has proved to be very useful for advancing the under- standing of the fracture topography of titanium alloys, including the relationship to the underlying microstructure. 146 3 Technological Aspects Fig. 3.87. Colony structure in Ti-6Al-4V resolving small secondary α phase plates and thin β phase “ribs” between the α phase plates, SEM (courtesy M. Juhas, The Ohio State University) Recently an improved capability to generate backscattered electron diffraction patterns and to automatically index them in real time with the aid of a computer has led to a new type of imaging [3.45]. This is called orientation imaging micros- copy (OIM). This imaging method collects the crystallographic orientation of individual microstructural constituents as determined in the SEM by backscattered electron diffraction and generates an image that depicts the variations in crystallo- graphic orientation. The image is generated with the use of an orientation polyhe- dron with surfaces that are mis-oriented from the specimen surface normal by preset amounts. For example, if the basal pole of an α grain is within 5° of the beam, the pixels belonging to this grain are presented in the image as one color. Grains that have a mis-orientation greater than this but less than, for example 10º, can be presented as another color. The resulting image is an orientation map, which gives complementary information to the actual image. The orientation polyhedron is shown in Fig. 3.90. Normally this polyhedron would have different colors for each face, but the different gray levels shown here provide a reasonable Fig. 3.88. Plateau etched specimen show- ing the relation of fracture topography to underlying microstructure, SEM Fig. 3.89. Plateau etched fatigue fracture surface showing the relation between secondary cracks and α plates, SEM 3.9 Characterization Methods 147 idea of this part of the technique. Figure 3.91 shows two orientation images gener- ated using this imaging technique. From this figure the large regions of consistent gray level in Fig. 3.91a is consistent with the presence of microtexture, while the more random distribution of gray shades in Fig. 3.91b is consistent with very little microtexture. As illustrated earlier, the use of polarized light can qualitatively provide the same information in a much shorter time. The use of OIM should be viewed as a quantitative complement to the polarized light method. Because of the cost of OIM facility and the time required to generate OIM images, it is recom- mended that polarized light be used before this more elaborate technique. In cubic materials where there is little or no optical anisotropy, OIM is useful in under- standing local orientation, for example where it is not clear whether boundaries are high angle grain boundaries or sub-boundaries. Fig. 3.90. Orientation polyhedron that allows interpretation of orientation images a b Fig. 3.91. Orientation images of Ti-6Al-4V: (a) Strong microtexture (b) Without significant microtexture (courtesy A. P. Woodfield, GE Aircraft Engines) 148 3 Technological Aspects 3.9.3 X-Ray Diffraction X-ray diffraction has been used to determine the structure and amount of phases in crystalline materials for a long time [3.46]. There are comprehensive texts on this technique, so there is little need to provide a detailed description of this technique here. Instead, there is one characteristic of titanium that merits discussion in con- nection with X-ray diffraction studies. The most useful wavelength for conducting X-ray diffraction studies of metals is Cu Kα because it provides a reasonable balance between penetration and diffraction peak resolution. However, titanium fluoresces under Cu Kα radiation that increases the background intensity in dif- fraction patterns obtained using this radiation. This increased background can mask low intensity peaks. There are two approaches to eliminating the effects of sample fluorescence. One is the use of a diffracted beam monochromator, which only allows the diffracted Cu Kα radiation to enter the detector. In such a mono- chromator, the most efficient monochromator crystal to use is pyrolytic graphite. This method works very well and allows acquisition of high quality patterns with good peak to background ratios. The other method is to use an energy dispersive detector so only the Cu Kα radiation is accepted by the detector. If this option is selected a detector with about 125 eV resolution is required for good results. Ev- erything else being equal, the diffracted beam monochromator is simpler and works very well. Because α titanium is hexagonal, solid titanium samples are seldom isotropic. Consequently, the relative intensities of the diffracted peaks seldom match the standard values for powder diffraction patterns. In the extreme, reflections can be missing altogether. The use of a specimen spinner helps for samples where the directionality lies in the plane of the sample. This is often the case for flat rolled products such as sheet and plate, but may not be the case for forgings, where the typically weak texture also has a different orientation. The point of mentioning this here is simply a caution to avoid drawing detailed conclu- sions from X-ray patterns obtained from solid specimens if these conclusions can be affected by the presence of texture, which is usually present in titanium mill products. X-ray diffraction also is used to determine pole figures used in describing the nature and amount of texture present in titanium and titanium alloy products. X- ray pole figures provide information regarding average preferred orientations in a volume of material. If needed, pole figures can be complemented by orientation image microscopy (see Sect. 3.9.2.2). The effects of texture on the properties of titanium alloys depend on both the type of texture, including the degree or inten- sity, and on the specific alloy. A recent book by Kocks treats this matter in detail [3.47]. Texture effects will be discussed in connection with individual alloy classes later in this book. Finally, X-ray diffraction is used to determine residual stresses resulting from processing of titanium alloys. This residual stress can shift the mean stress level in a component and affect fatigue behavior. It also can lead to stress assisted migra- tion and concentration of hydrogen and can cause the occurrence of stress corro- sion cracking where it might not otherwise be expected. Measurement of residual stress by X-ray diffraction has been performed for years on a routine basis for 3.9 Characterization Methods 149 cubic metals such as aluminum alloys and steel. The elastic and plastic anisotropy of α titanium and the potential for crystallographic texture makes the reduction of X-ray diffraction data into residual stress components more difficult. Conse- quently, the reader interested in measuring residual stresses in α+β titanium alloys must be prepared to spend some time in planning the residual stress measurements and in analyzing the results. 3.9.4 Mechanical Testing The purpose of this section is to call attention to aspects of mechanical testing that are peculiar to titanium. The section is not intended to be an exhaustive discussion of mechanical testing per se. Such discussions can be found in a variety of books on this subject, see for example [3.48, 3.49]. The preparation of titanium and titanium alloy fatigue specimens for endurance testing must be done very carefully, because, titanium is not a hard material like steel. The specimens require mechanical polishing after machining to obtain a reasonable surface finish that also is free of any residual stresses due to machining or subsequent polishing. Clearly, surface residual stresses will affect the initiation of fatigue cracks and interfere with obtaining true fatigue lives. Residual scratches also can behave as notches so mechanical polishing should be done in the longitu- dinal direction to avoid any scratches (notches) oriented normal to the loading axis. Laboratory fatigue specimens are often electrolytically polished to provide a maximum consistency in surface quality and minimize the scatter due to this fac- tor. For research purposes, this is a useful approach. For the generation of fatigue data, mechanical polishing is a better approximation to the surfaces of titanium components that would be used in manufactured products. In these cases longitu- dinal mechanical polishing gives data that approaches the behavior of such com- ponents in service. The goal is to generate data that is neither conservative nor non-conservative. There are several companies that specialize in specimen prepa- ration and testing which provide consistent results that meet this description. It is not surprising that these companies often are located in the proximity to aircraft or aero-engine companies. Figure 3.92 shows the variation in fatigue life that can be obtained due to different specimen preparation methods, such as rough mechanical polishing, electrolytic polishing, fine mechanical polishing, and shot peening [3.50]. If the electrolytically polished curve is taken as the “true” fatigue capabil- ity of the material, it becomes clear that specimen preparation methods and sur- face condition can lead to either better or poorer fatigue life. The test environment also plays a significant role in determining the fatigue re- sults, In this case, testing in inert gas, vacuum, and air of varying relative humid- ity, all affect the life. Examples on the influence of environment on mechanical properties will be shown in Sects. 5.2.6 and 6.2. It is important to be aware of this point when examining the literature and if self-consistent results are to be ob- tained. Hydrogen that is present in the material as a residual impurity or that may have been introduced during specimen preparation also can have a negative effect on the measured fatigue life. Thus, the precautions outlined in Sect. 3.9.2.1 regarding [...]... produce metallic titanium This objective turns out to be a significant challenge, which is not surprising when the stability of rutile is considered Achieving lower cost titanium is further complicated by the relatively small consumption of titanium tetrachloride (TiCl4) by the titanium industry compared to other products (e.g paint) that use this chemical This situation results in the titanium producers... the particular feature of the process has on the overall process economics These new processes will be discussed using the following characteristics: • Processes that aim to produce molten titanium metal These are a subset of all the electrolytic processes and among them are the Ginatta [3.54], CSIR (South Africa), Rio Tinto [3.55], and MIT [3. 56] processes • Processes that produce solid titanium in particulate... creep testing titanium, considerably more care must be exercised to obtain reliable data compared to that required for Ni base alloys Fig 3.92 S-N curves for Ti-6Al-4V showing the effect of specimen preparation and surface condition on fatigue life 3.10 Recent Developments since the First Edition 3.10.1 New Titanium Production Methods The biggest barrier by far to the introduction of titanium alloys... jackets However, the transfer of molten titanium from an electrolytic cell to a casting furnace is more challenging than it may appear due to the high melting temperature ( 167 0°C) and the extreme reactivity of molten titanium It appears that this idea may have been fashioned after the practice in the aluminum die casting industry where molten aluminum alloys at about 65 0°C are taken directly from the smelter,... it removes several steps from the conventional process for making titanium ingots by VAR methods In particular, pre-alloyed particulate that can be directly consolidated into an intermediate mill product eliminates the need for blending and compaction of the sponge and master alloy, the creation of a first melt VAR electrode (Fig 3 .6) , preliminary melting into an ingot (Fig 3.7), and the conditioning... such as titanium rich slag as the feedstock These processes have the cost advantage of not using TiCl4 as a feedstock Central to each of these processes is an electrolytic cell containing fused salt, a cathode made up of or containing the titanium bearing feedstock and an anode, usually made of carbon The exception is the MER process which uses a steel or other metal cathode where the titanium particulate... streak is shown in Fig 3.96a Some of these particles and the W enriched β regions are shown in the back scattered electron SEM image in Fig 3.96b Energy dispersive X-ray analysis has been used to unequivocally identify the bright particles as W For low temperature applications theses small W-rich inclusions are probably benign because of the low diffusivity of W in titanium However, if pieces of material... of titanium is only an experimental process at present but if it were to gain acceptance this could become an issue 3.10 Recent Developments since the First Edition 157 a b Fig 3. 96 Micrographs showing small particles of W tool debris embedded in the material during FSP, SEM BSE: (a) Low magnification showing extent of a W-rich streak (b) Higher magnification showing details of streak including W particles... regarding nature of the pre-alloyed particulate product Included are the amount of residual NaCl entrapped in the product and the ability to realize pre-alloyed particulate with consistent composition control These matters are among the primary points of investigation as the processes become more mature Finally, these processes offer the opportunity to add alloying elements to titanium that would be difficult,... fused halide salts containing CaCl2 These processes also produce solid titanium product which can be readily converted to particulate Included in this group are the FFC (also called EDO, see Sect 3.1), OS [3.57], BHP Billiton, MER, and EMR/MSE [3.58] processes The first group of processes, those that produce or aim to produce molten titanium directly, is attractive in principle because these processes . 3. 86. Example of thin foil artifact: “Martensite-like” features in metastable β phase, Ti- 62 46, TEM 3.9.2.2 Scanning Electron Microscopy The intrinsically fine microstructural scale of titanium. difficulty encountered in preparing high quality thin foils of titanium base materials. This was in part related to the reactiv- ity of titanium including its propensity to absorb hydrogen during. characterization of titanium and its alloys including a brief summary of the relevant experimental techniques. 3.9.2.1 Transmission Electron Microscopy The ability to examine titanium and titanium alloys